Spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. Alternatively, two rods can be disposed on the lateral or anterior surface of the vertebral body in a substantially parallel relationship. The fixation rods can have a predetermined contour that has been designed according to the properties of the target implantation site and, once installed, the rods hold the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Spinal fixation devices can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a receiving member having a U-shaped slot for seating the fixation rod. The receiving member can be monoaxial and thus fixed relative to the threaded shank, or it can be polyaxial and thus movable relative to the threaded shank. Polyaxial screws can facilitate positioning of the fixation rod therein. Extension members are often coupled to the receiving member, especially in minimally invasive procedures, to provide a pathway through tissue to the receiving member. A set-screw, plug, or similar type of closure mechanism, is used to lock the fixation rod into the rod receiving member of the pedicle screw.
While current spinal fixation systems have proven effective, difficulties are still encountered in various spinal procedures, such as when correcting spinal deformities. For example, the use of polyaxial screws in these operations can aid in capturing a rod or other spinal fixation element within the receiving member of the polyaxial screw due to the ability of the receiving member to move relative to the threaded shank implanted in the patient's vertebra. However, the movement provided by polyaxial screws can limit a surgeon's control when applying corrective forces to the screw in order to effect movement of the vertebra. Various devices exist to lock a polyaxial screw in a monoaxial configuration, but these devices can be problematic as well because surgeons often cannot tell when the receiving member is correctly oriented with respect to the threaded shank implanted within the vertebra. In particular, locking the polyaxial screw in a monoaxial configuration when the receiving member is angled relative to the threaded shank can create large moment forces on the screw during the application of corrective forces. To combat these forces, surgeons often want to lock the polyaxial screw in a monoaxial configuration when the receiving member is aligned with the threaded shank (i.e., the longitudinal axes of the receiving member and the threaded shank are coaxial). Because there is not an easy and cost-effective way to align a polyaxial screw in a coaxial configuration, surgeons often utilize various combinations of polyaxial, monoaxial, and uniplanar screws (the latter provides relative motion between the receiving member and threaded shank in only a single plane).
The use of multiple screw types, however, can be problematic because they add to the complexity of an already technically challenging procedure. Furthermore, beyond the addition of the screws themselves, the use of additional screw types can require that additional instrumentation be present in the operating room as well. Surgeons may need additional training on the use of the different screw types and their associated instrumentation, and costs associated with sterilizing and maintaining the instrumentation and implants are also increased. Still further, monoaxial and, to a lesser degree, uniplanar screws lack the ability to conform to a rod or other spinal fixation element, which can increase the difficulty of capturing and approximating a rod or other spinal fixation element during a procedure.
Accordingly, there is a need in the art for methods and devices that allow surgeons to utilize polyaxial screws in a wider range of surgical procedures. In particular, there is a need for methods and devices that allow for rod capture via polyaxial movement of a screw receiving member while also allowing a surgeon to selectively lock the receiving member in coaxial alignment with an implanted shank after rod capture.